1. Field of the Invention
The present invention relates to an adhesive sheet and a release sheet, particularly to an adhesive sheet and a release sheet for an electronic material, which are used in a process for producing a precision electronic device such as a ceramic capacitor, hard disk drive, or semiconductor device.
2. Description of the Related Art
Conventionally, it is known that an adhesive sheet is constituted of an adhesive base sheet, an adhesive layer, and a release sheet laminated together. In particular, the adhesive sheet is widely used with electronic materials, for example, in the production process for a precision electronic device such as a ceramic capacitor, hard disk drive, or semiconductor device.
The adhesive sheet for the electronic material is often bonded to a circuit surface of a wafer in a process for grinding the wafer in order to protect the circuit surface of the wafer from contamination. At this point, the release sheet is peeled away and then the adhesive sheet is bonded to the wafer while the adhesive sheet is being delivered from one roll to another; hence the adhesive sheet stretches when it is bonded to the wafer. The stretching adhesive sheet shrinks following bonding to the wafer, causing the wafer to warp, particularly when the stretching of the adhesive sheet is acute. In addition, the recent development of thinner wafers may be exacerbating the warping problem. Therefore, it is necessary to bond the adhesive sheet with a limited amount of stretching to the wafer.
The high release force of the release sheet makes the adhesive sheet stretch substantially when the release sheet is peeled away, and hence may cause the wafer to warp. Therefore, it is necessary to utilize a low release force in order to reduce the warp of the wafer.
The adhesive sheet is wound up into a roll-like shape and kept in storage before use. If the release force is low, however, a part of the release sheet may be released by itself from the adhesive sheet (namely “tunneling” occurs) while the rolled-up adhesive sheet is kept in storage.
A non-silicone release agent is utilized in the adhesive sheet used in a process for producing a precision electronic device conventionally. The non-silicone release agent prevents the silicone deposits from contaminating the electronic components, thus reducing the potential of defect in the electronic device.
The general non-silicone release agent does not release in a stable manner with respect to time when the release force is low; hence, the release force thereof should generally be made to be high. However, when 1,4-polybutadiene is utilized as the release agent, the release sheet maintains a low release force that releases in a stable manner with respect to with time, as shown in Japanese Unexamined Patent Publication (KOKAI) NO. 2005-205813 (hereinafter “Document 1”).
In addition, as shown in Japanese Unexamined Patent Publication (KOKAI) NO. 2004-182836 (hereinafter “Document 2”), it is known that the releasing sheet having a releasing agent layer containing a gas generating agent in response to a stimulus is used in a process for producing a precision electronic device. In this release sheet, it is possible to decrease the release force by generating a gas in response to the stimulus.
However, even when 1,4-polybutadiene is utilized as the release agent, as shown in Document 1, the release force is still not low enough to reduce the warp of the wafer. Further, even if the release force is reduced by utilizing the silicone release agent in order to reduce the warp of the wafer, it is impossible to maximize the release force under storage conditions, thus making “tunneling” unavoidable. Furthermore, Document 2 discloses that the release force is variable by providing stimulus, but it does not disclose the presence of an expandable fine particle or a gas generating fine particle within the release sheet.